Backfiring (or more accurately in this situation "afterfiring") is caused by unburned fuel/air mixture being ignited by the heat of the muffler.

When the ignition switch is turned to the OFF position, the spark to the spark plug stops immediately, although the engine continues to rotate for a short while afterwards. As a result of this engine rotation, more air/fuel mix is pulled through the carburetor. Because there is no spark this fuel is not ignited in the combustion chamber as it normally is. This mixture can then build up in the hot muffler and ignite. The loud bang heard is the sound of this fuel igniting after the engine has shut off.

Allowing the engine to idle from 30-60 seconds before turning the key to the OFF position will generally prevent this afterfire condition.

Engines equipped with an electronic fuel shutoff solenoid have an extra valve that shuts off the fuel system immediately when the ignition is stopped. These models should be turned OFF with the engine at FULL throttle. The length of time it takes for these engine to come to a complete stop is generally long enough to flush out the residual exhaust mixture, but there is no raw fuel to ignite in the muffler.

There are several other factors involved such as; ambient temperature, fuel additives, carburetor settings, alcohol enriched fuels, etc.. that may also contribute to the afterfire condition. These are only suggestions, a service dealer will have to make the final determination as to the exact cause and best remedy for each individual engine design and application.